Question

When lemon juice is added to milk, it curdles. This is an example of:
(A) Coagulation
(B) Sublimation
(C) Chemical decomposition reaction
(D) Photochemical change

Answer 1

Hint: - Coagulation refers to the process of aggregating together the colloidal particles so as to convert them into large sized particles which ultimately settle as precipitate.
- Milk primarily consists of fat, protein, lactose (a kind of sugar) and water. The proteins are mostly casein proteins. One end of casein proteins likes to be surrounded by water and hates to be surrounded by fats while the other end hates to be surrounded by water and likes to be surrounded by fats.
- In milk, these casein proteins form tiny spheres called micelles, with the water –hating ends facing inwards and the water-loving ends facing outwards into the water.

Complete step by step answer:
Each end of the casein protein which is sticking into water grabs electrons and attains negative charge. Thus, all the micelles are negatively charged and so they repel one another which prevent them from aggregating or clumping together.
Now, an acid liberates positive hydrogen ions in water. So when lemon juice which contains citric acid is added to milk, the positive hydrogen gets attracted towards the negative micelles and they are neutralized. Thus, all the micelles will now come close together and aggregate together to form a gelatinous material called curd. When the lemon juice is added into water, the milk becomes sour and bacteria convert the sugar lactose into lactic acid which does the curdling.
Thus, curdling of milk is due to coagulation of the casein micelles. So, the correct option is A.
Since sublimation is a phase transition of solid directly into vapour without passing through the liquid phase, it is not the reason for milk curdling and so option B is wrong.
Chemical decomposition is the breakdown of a single chemical entity into two or more fragments and a photochemical change is initiated by absorption of energy in the form of light. So, these two processes are not related to curdling and hence options C and D are wrong.

Note: Milk curdling can also be done by letting the milk age long enough or by treating the milk with enzymes. For example, chymosin enzyme alters the casein micelle structure to make the milk curdle. Protease is another class of enzyme that breaks up proteins into fragments thereby disrupting the micelle structure and making the milk curdle.